This invention relates to the preparation of a calcium fluoride containing solid from fluosilicic acid (H.sub.2 SiF.sub.6) solutions. More particularly, the invention is related to preparing a calcium fluoride which does not contain significant amounts of free silica, e.g. silica gel or a silica sol, which could interfere with the use of the calcium fluoride in treating impure phosphoric acid for the removal of contaminants.
U.S. Pat. No. 2,780,523 to Gloss describes the preparation of calcium fluoride from weak solutions of fluosilicic acid (less than 4% H.sub.2 SiF.sub.6) which are derived from scrubbing waste gas streams with water. Precipitation is done by adding calcium carbonate in two stages: first, neutralizing about 85% of the acid with CaCO.sub.3, recovering a solid product with a low silica content (less than 1.5%), followed by a second addition of CaCO.sub.3 to a pH of greater than 7, which yields a solid mixture of calcium fluoride and calcium silicofluoride. Evaporation of the filtrate then yields a low fluoride silica gel product. A temperature range of 60.degree.-90.degree. F. is preferred to avoid contamination of the calcium fluoride from the first precipitation with silica.
U.S. Pat. No. 2,914,474 to Hillyer et al. is for a process to remove fluorine as calcium fluoride from industrial wastewaters in which at least a portion of the fluorine is present as fluosilicate. In this process, an acidic fluorine-containing solution is neutralized with calcium carbonate to a pH of at least 5, and an excess of calcium added, as calcium sulfate, to precipitate calcium fluoride. A temperature in the range of 70.degree.-200.degree. F. is preferred, but higher temperatures are not recommended for this process.
U.S. Pat. No. 3,549,317 to Dorn et al. teaches a process for the preparation of a silica filler having a particular surface area and a calcium fluoride from fluosilicic acid or sodium fluosilicate, present at about 10 to 40% by weight in aqueous medium. The fluosilicate is reacted with calcium carbonate at a temperature above 50.degree. C., preferably 50.degree. to 60.degree. C., until no further evolution of CO.sub.2 is observed, forming calcium fluoride and a silica material. The silica is dissolved in sodium hydroxide at temperatures of 50.degree. C. to the boiling point of the mixture, and calcium fluoride removed by filtration. The sodium silicate solution which remains is treated with acid, maintained at a temperature above 90.degree. C. for some period of time, and the precipitated silicia separated by filtration.
U.S. Pat. No. 3,551,332 to Baumann et al. describes a purification treatment for fluorine-containing wastewaters having a pH less than 3. Calcium carbonate is added to produce a pH of 3 to 3.3, yielding a precipitate of calcium fluoride. The filtrate is then treated with lime to a pH greater than 7 (preferably 11 to 12), and additional filtrate added to give a pH of 5.5 to 7. After a settling period, the solids are removed and discharged. An improved process results from the presence of sulfate or phosphate ions in the wastewater. A temperature in the range of 35.degree.-130.degree. F. is useful, with temperatures of 60.degree.-90.degree. F. preferred.
U.S. Pat. No. 3,907,978 to Spreckelmeyer is for the production of a calcium fluoride which has a low silica content. A suspension of calcium carbonate in water is prepared and a stoichiometric deficiency of fluosilicic acid reacted with the suspension. After removing the calcium fluoride which forms (and the unreacted CaCO.sub.3), it is treated with additional fluosilicic acid to produce a more pure calcium fluoride. To obtain the desired product, the initial suspension should contain three to five parts of water per part of CaCO.sub.3. A temperature of 0.degree.-30.degree. C. is preferred, since silica tends to precipitate at temperatures above 40.degree. C.
U.S. Pat. No. 4,031,193 to Becker et al. discloses a process for preparing calcium fluoride, which is suitable for producing hydrofluoric acid, from a fluosilicic acid. A suspension of calcium carbonate and water (with added sulfate or aluminum ions to promote the desired reaction) is reacted with fluosilicic acid, preferably maintaining a pH of 4 to 5 during the reaction. After the reaction, additional H.sub.2 SiF.sub.6 is added to yield a pH of 3 to 3.5. A precipitate which contains mostly calcium fluoride is obtained, and the filtrate contains most of the silicon as a silicic acid sol. The preferred temperature ranges is 5.degree. to 30.degree. C., although temperatures in excess of 30.degree. C. may be employed.
U.S. patent application Ser. No. 863,085, filed Dec. 22, 1977, an invention of Mills and Hirko, which is incorporated herein by reference, is directed toward the recovery of calcium fluoride from pond waters resulting from phosphoric acid processing. The pond water is treated in a first stage with calcium carbonate, in an amount of between 0.3 and 0.8 equivalents of CaCO.sub.3 per equivalent of fluorine, and then treated in a second stage with additional CaCO.sub.3 at no less than 0.8 equivalents per equivalent of fluorine. A solid cake results, and this is treated with a mineral acid and, finally, a water wash to give a calcium fluoride with up to 45% fluorine. The acid and water washes of the solid cake are combined with the treated waste water effluent from the second stage in a third stage, where lime is added and the resulting calcium fluoride filtered out, and the third stage filtrate is carried to additional lime precipitation stages.
The first and second stage reactions can be conducted at temperatures between the freezing point and the boiling point of the pond water, and calcium oxide may be used in place of calcium carbonate. A calcium fluoride results from the process of sufficient purity for the preparation of hydrofluoric acid (less than 5 to 10% P.sub.2 O.sub.5 and less than 5% SiO.sub.2).
Solids which contain calcium fluoride have been used in processes for the purification of wet process phosphoric acid, as in U.S. Pat. No. 4,136,199 to Mills, which is incorporated herein by reference. The calcium fluoride-containing product obtained from pond water by the process of U.S. patent application Ser. No. 457,565, filed Apr. 3, 1974 (now abandoned in favor of Ser. No. 756,009 filed Jan. 3, 1977 which was abandoned in favor of Ser. No. 840,553, filed Oct. 11, 1977, of which the above referenced Ser. No. 863,085 is a continuation-in-part) may be added to impure wet process phosphoric acid and the mixture held at an elevated temperature until a complex compound (which has been found to resemble Ralstonite, Na.sub.x Mg.sub.x Al.sub.2-x (F,OH).sub.6 'H.sub.2 O) forms by removing magnesium, aluminum, etc. from the phosphoric acid. Upon filtration to separate the precipitated compound, a purified phosphoric acid is obtained.
U.S. patent application Ser. No. 954,647, an invention of Mills, filed Oct. 25, 1978 (a continuation-in-part of the application which became U.S. Pat. No. 4,136,199 above), which is incorporated herein by reference, further describes the conditions which improve the purification process.
The process of this invention produces a calcium fluoride-containing mixture from fluosilicic acid which can be used in the above described purification of phosphoric acids, but does not require the lengthy silica separation steps previously described, since the silicon present in the fluosilicic acid is rendered substantially inert to phosphoric acids, and therefore does not inhibit the fluorine-contributing ability of the calcium fluoride product which is needed for phosphoric acid purification. Other advantages of the invention include the ability to use less expensive materials in the fabrication of process equipment, since calcium fluoride is formed in an alkaline medium (not the acidic medium for the previously described conventional processes) and the simplicity of separating the suspended calcium fluoride-containing solids which are formed, since there is no silica gel or sol (which impedes filtration) present in the suspension.
The inherent simplicity of conducting the process of this invention will become apparent from the following discussion and examples, and should be compared to the lengthy, multi-step procedures and complex process equipment required in the references previously noted.